Superabsorbent resin composition

ABSTRACT

A superabsorbent resin composition comprising the following components (A), (B) and (C), wherein 
     (A) is a superabsorbent resin, 
     (B) is a metal compound containing at least one metal A selected from the group consisting of titanium and zirconium and 
     (C) is a chelating agent.

BACKGROUND OF THE INVENTION

1 Field of the Invention

The present invention relates to a superabsorbent resin composition.

2 Description of Related Art

Superabsorbent resins have been widely used as a water-absorbingmaterial in absorbent articles in the sanitary field, such as disposablediapers for babies, adults or those suffering from incontinence, andsanitary napkins. It is known that water-soluble polymers (crosslinkedpolymers) constituting such superabsorbent resins undergo molecularweight reduction (degradation) and deterioration with time in thepresence of radical generating species, such as hydrogen peroxide orL-ascorbic acid or a salt thereof In particular, where thesuperabsorbent resin is used in absorbent articles such as disposablediapers and sanitary napkins, because L-ascorbic acid or a salt thereofis present in body fluids, such as urine, blood, and perspiration, ithas been a serious problem that a superabsorbent resin used in suchabsorbent articles undergoes degradation and deterioration with time dueto the radicals generated from L-ascorbic acid or a salt thereof andreduces its capacity of retaining a body fluid.

The degradation reaction of a water-soluble polymer due to such radicalgenerating species is conspicuous after the polymer has absorbed anaqueous liquid or a body fluid, such as urine, blood or perspiration(hereinafter referred to as a water-containing condition), especially inthe co-presence of transition metal ions capable of having more than oneoxidation number, such as iron ions or copper ions, in the air.

Known approaches for inhibiting the above-described degradation anddeterioration of superabsorbent resins include (1) a method comprisingsealing the superabsorbent resin under reduced pressure, or in anitrogen atmosphere so as to avoid contact with air (especially oxygen),(2) a method comprising using highly purified water or raw materials soas to prevent metallic ions from entering the superabsorbent resin, (3)a method comprising adding an antioxidant or a reducing agent to thesuperabsorbent resin, (4) a method of adding proteins or enzymes to thesuperabsorbent resin, and (5) a method of adding to the superabsorbentresin, metal chelating agents, such as citric acid, (poly)phosphoricacid or a salt thereof, and ethylenediaminetetraacetic acid (EDTA) or asalt thereof.

However, the methods (1) and (2) are in many cases practicallyimpossible to apply depending on the use of the superabsorbent resin.Methods (3), (4), and (5) that rely on conventional additives achievesome effect in suppressing degradation and deterioration ofsuperabsorbent resins but are not always sufficient in effect. In manycases, an additive must be added in a large quantity or an additiveexerting a very strong action must be used before the desired effect canbe manifested. Under such circumstances, the essential physicalproperties or functions of the superabsorbent resin tend to be seriouslyruined.

According to the inventors' study, it has turned out that the use ofEDTA or sodium tripolyphosphate is not so effective in stabilizing asuperabsorbent resin in the presence of an aqueous solution or watercontaining a radical generating species, e.g., hydrogen peroxide orL-ascorbic acid or a salt thereof.

In addition to the above-mentioned performance properties ofsuperabsorbent resins, i.e., stability over time in a water-containingstate (gel stability with time), the water absorption capacity (theamount of water absorbed), the rate of water absorption, the gelstrength after swelling, liquid permeability, and the like are importantrequirements for superabsorbent resins. However, many of theseproperties conflict with each other, and it is very difficult to meetall of these requirements, which has been one of the problems indeveloping superabsorbent resins. For example, an attempt to increasewater absorption capacity is generally accompanied by reductions in gelstrength after swelling and liquid permeability.

In order to solve these problems, for example, Japanese Patent Laid-OpenNo. 36411/87 proposes graft-treating a carboxyl- and/orcarboxylate-containing superabsorbent resin with a silane couplingagent. Japanese Patent Laid-Open No. 306118/94 proposes treating asuperabsorbent resin with a titanium alkoxide. Nevertheless, thesemethods are still insufficient for satisfying both superabsorbentperformance (e.g., a water absorption capacity) and gel stability withtime after swelling.

Japanese Patent Laid-Open No. 145326/95 discloses the addition of asulfate of a polyvalent metal selected from titanium, zirconium andvanadium to a superabsorbent polymer as one method for simultaneouslyimproving gel strength, stability, and stickiness after waterabsorption.

According to the inventors' study, however, the gel stability over timeachieved by this method is insufficient particularly for polymers havinghigh water absorption capacity. Besides, addition of a polyvalent metalsulfate tends to reduce the initial rate of water absorption of thesuperabsorbent polymer or tends to make the polymer particles beforewater absorption ready to agglomerate due to the hygroscopicity of thepolyvalent metal sulfate.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a superabsorbent resincomposition which exhibits high water absorption capacity and is yetstable against degradation and/or deterioration even in the presence ofan aqueous solution or water containing a radical generating species,such as L-ascorbic acid or a salt thereof, or a transition metal ion,such as an iron ion or a copper ion.

The present invention provides a superabsorbent resin compositioncomprising following components (A), (B) and (C), or components (A) and(D):

(A) a superabsorbent resin,

(B) a metal compound containing at least one metal (referred to as metalA hereinafter) selected from the group consisting of titanium andzirconium,

(C) a chelating agent, and

(D) a coordination compound in which component (C) is coordinated withmetal A.

The superabsorbent resin composition of the present invention isexcellent in that it has high water absorption capacity and yet thesuperabsorbent resin used therein does not undergo degradation ordeterioration even in the presence of an aqueous solution or watercontaining radical generating species, such as L-ascorbic acid or a saltthereof, or transition metal ions, such as iron or copper ions.

DESCRIPTION OF THE EMBODIMENT

The superabsorbent resin which can be used in the present invention ascomponent (A) is not particularly limited and includes, for example,partially crosslinked polymers containing a carboxyl group or a saltthereof, such as a crosslinked polyacrylic acid salt, a crosslinkedpoly(vinyl alcohouacrylic acid salt) copolymer, a crosslinkedstarch-acrylic acid salt graft copolymer, and a crosslinked polyvinylalcohol-polymaleic anhydride salt graft copolymer; and partiallycrosslinked polysaccharides, such as a crosslinked carboxymethylcellulose salt. A crosslinked polyacrylic acid salt or a crosslinkedstarch-acrylic acid salt graft copolymer are preferred for their highwater absorptivity. A crosslinked polyacrylic acid salt is the mostpreferred.

These superabsorbent polymers can be used either individually or as acombination of two or more thereof A “salt” of the superabsorbent resinsillustrated above includes an alkali metal salt (e.g., sodium salt,potassium salt or lithium salt), an alkaline earth metal salt (e.g.,calcium salt, magnesium salt or barium salt), and an ammonium salt(e.g., a quaternary ammonium salt or a quaternary alkylammonium salt).

The superabsorbent resin preferably has a degree of neutralization of0.01 to 100%, still preferably 1 to 99%, particularly preferably 40 to95%, based on the number of moles of the acid radical in thesuperabsorbent resin.

The terminology “degree of neutralization” as used herein denotes amolar ratio of acid radicals in a salt form to the total acid radicalsof a superabsorbent resin, i.e., (the number of moles of acid radicalsin salt form)/(the total number of moles of free acid radicals capableof forming a salt and acid radicals in salt form)×100 (%).

The metal compound which can be used in the present invention ascomponent (B) is a compound containing at least one metal selected fromthe group consisting of titanium and zirconium (hereinafter referred toas metal A).

The metal compound as component (B) preferably includes the followingcompounds (B-1) to (B-6).

(B-1) Compounds obtained by mixing a hydroxy acid or a salt thereof witha polyvalent metal salt comprising at least one metal A selected fromthe group consisting of titanium and zirconium or an alkoxide of metalA.

(B-2) Compounds obtained by hydrolyzing a polyvalent metal saltcomprising at least one metal A selected from the group consisting oftitanium and zirconium or an alkoxide of metal A in the presence of ahydroxy acid or a salt thereof

(B-3) Titanium dioxide.

(B-4) Water-containing metal oxides comprising at least one metal Aselected from the group consisting of titanium and zirconium and atleast one metal selected from the group consisting of zinc, aluminum,calcium, magnesium, and silicon (hereinafter referred to as metal B).

(B-5) Titanium alkoxides.

(B-6) Sulfates of at least one metal A selected from the groupconsisting of titanium and zirconium.

Compound (B-1) is obtained by mixing a hydroxy acid or a salt thereofand a polyvalent metal salt comprising at least one metal A selectedfrom the group consisting of titanium and zirconium or an alkoxide ofmetal A.

The hydroxy acid is a compound having a hydroxyl group and a carboxylgroup per molecule and is not particularly limited in kind. Examples ofsuitable hydroxy acids are α-hydroxy acids. Where compound (B-2) isobtained by hydrolyzing component (B-1) as hereinafter described, it isdesirable for the hydrolyzate be water-soluble. Accordingly,water-soluble hydroxy acids are preferred. Water-soluble α-hydroxy acidsare still preferred. Examples of such α-hydroxy acids are gluconic acid,citric acid, isocitric acid, alloisocitric acid, lactic acid,hydroxyacetic acid, malic acid, and tartaric acid, with gluconic acidand citric acid being particularly preferred.

Examples of salts of the above-enumerated hydroxy acids include alkalimetal salts (e.g., sodium salt, potassium salt, and lithium salts),alkaline earth metal salts (e.g., calcium salt, magnesium salt, andbarium salt), and ammonium salts (e.g., quaternary ammonium salts andquaternary alkylammonium salts).

These hydroxy acids and salts thereof can be used either individually oras a mixture of two or more thereof

The metal A is one or both of titanium and zirconium. In other words,titanium and zirconium can be used either alone or in combination.Titanium is preferred as metal A from the standpoint of the degree ofimprovement attained and economy.

The polyvalent metal salt made of metal A is not particularly limitedand includes a sulfate, an oxysulfate, a chloride, an oxychloride, anitrate, an oxynitrate, and a carboxylate of metal A. A sulfate, anoxysulfate, a chloride, and an oxychloride are preferred, with a sulfateand a chloride being still preferred.

The alkoxide of metal A includes a tetraisopropoxide and a tetrabutoxideof metal A.

When a hydroxy acid or a salt thereof and a polyvalent metal salt orpolyvalent metal alkoxide are mixed, they are preferably mixed in theform of an aqueous solution or an alcoholic solution. In particular,where a polyvalent metal salt is used, they are preferably mixed in theform of an aqueous solution, and where a metal alkoxide is used, theyare preferably mixed in the form of an alcoholic solution. When mixed asan aqueous solution or an alcoholic solution, the system provides a moreeffective compound.

The hydroxy acid or a salt thereof is preferably used in an amount ofnot less than twice the molar quantity of metal A in the polyvalentmetal salt or alkoxide. If the molar ratio is less than 2, the system,i.e., a mixed solution, tends to become heterogeneous, which isunfavorable for the effects and handling properties discussed above.

It is preferable that the hydroxy acid or a salt thereof and thepolyvalent metal salt or alkoxide be not only mixed but that the latterbe hydrolyzed in the presence of the former. In this case, a moreeffective metal compound is obtained.

Compound (B-2) is a compound obtained by hydrolyzing a polyvalent metalsalt comprising at least one metal A selected from the group consistingof titanium and zirconium or an alkoxide of metal A in the presence of ahydroxy acid or a salt thereof. That is, compound (B-2) is obtained byhydrolyzing compound (B-1). Therefore, the description about compound(B-1) hereinafter given will apply to compound (B-2) with respect to thekinds and amounts of the compounds used for obtaining compound (B-2) andthe like.

Various techniques of hydrolysis can be employed. For example, a base,such as sodium hydroxide, potassium hydroxide, ammonia, amine, etc., isadded, and, if necessary, the system is heated.

An illustrative example of the hydrolysis conditions is as follows. Thebase is preferably added in an amount 2 to 4 times the molar quantity ofthe metal A. The temperature of heating, if conducted, is preferably 60to 100° C. The hydrolyzing time is preferably 20 to 60 minutes. Water,preferably ion-exchanged water, is used in an amount of 100 to 1000parts by weight per 100 parts by weight of the polyvalent metal salt orpolyvalent metal alkoxide.

It is preferable for the hydrolyzate, which contains compound (B-2), tobe water-soluble. If the hydrolyzate has low water solubility and has ahigh insoluble content, the effects and handling properties are poor.

While the state of the resulting compound (B-1) or (B-2) on use in thepreparation of a superabsorbent resin composition hereinafter describedis not particularly limited, compound (B-1) is preferably used in theform of a solution as obtained by mixing in the form of an aqueous oralcoholic solution, and compound (B-2) is preferably used in the form ofa metal compound solution as obtained by hydrolysis. The metal compoundsolution preferably has a metal A content of 0.05 to 5% by weight,particularly 0.2 to 2% by weight.

Titanium dioxide used as compound (B-3) is not particularly limited inshape and size but preferably has an average particle size of 0.1 μm orsmaller, particularly 0.05 μm or smaller, especially 0.03 μm or smaller,and a specific surface area of 50 m²/g or more, particularly 100 m²/g ormore, especially 200 m²/g or more, as measured by aBrunauer-Emmett-Teller method (hereinafter referred to as a BET specificsurface area).

While the crystal form of titanium dioxide is not limited either, ananatase-type structure is preferred to a rutile-type structure formanifestation of higher effects of the present invention.

Compound (B-4) is a water-containing metal oxide comprising at least onemetal A selected from the group consisting of titanium and zirconium andat least one metal B selected from the group consisting of zinc,aluminum, calcium, magnesium, and silicon. While not limiting, thewater-containing metal oxide is preferably in a finely powdered state,being an aggregate of the water-containing metal oxide particles.

The terminology “water-containing metal oxide” as used herein means ahydrated oxide, that is, a hydrate of a metal oxide containing ahydroxide.

The water-containing metal oxide as compound (B-4) is a water-containingoxide containing a —M¹—O—M²— bond (wherein M¹ is metal A, and M² ismetal B) in at least part of its structure, which is different from amere mixture of a water-containing oxide of M¹ and a water-containingoxide of M².

While any combination of metals A and B is effective, it is preferredfrom the standpoint of the degree of improvement attained and economythat titanium be used as metal A and zinc or aluminum, especially zinc,be used as metal B. In other words, a combination of titanium and zincor a combination of titanium and aluminum is preferred.

The molar ratio of metal A to metal B in the water-containing metaloxide preferably ranges from 30/70 to 99/1, particularly 40/60 to 90/10.Out of the above molar ratio, the effect of combining metal A and metalB tends to be lessened.

It is preferred for the water-containing metal oxide to be amorphous asexamined by X-ray diffractometry or the like crystal structureanalytical methods. Amorphous water-containing metal oxide is moreeffective in suppressing degradation and deterioration of thesuperabsorbent resin even in a water-containing condition, i.e., insecuring the stability of the superabsorbent resin with time.

The water-containing metal oxide can be obtained through variousmethods, such as a liquid phase method, a vapor phase method, and asolid phase method. From the standpoint of equipment and productioncost, a liquid phase method, particularly a coprecipitation method isdesirable. A coprecipitation method is generally a method in which twoor more kinds of ions are precipitated simultaneously, i.e.,coprecipitated. The coprecipitation method as referred to in the presentinvention is a method in which two or more kinds of ions in a mixedsolution are coprecipitated by changing the concentration, pH,temperature, solvent, etc. of the mixed solution to obtain acoprecipitate having a prescribed composition, and the coprecipitate isseparated and dried. Therefore, it is different from a method in whichtwo or more kinds of metal ions are separately precipitated, collected,and dried, and the resulting precipitates are merely mixed together.

In the above coprecipitation method, coprecipitation can be induced byvarious techniques with no particularly restriction. For example,aqueous ammonia or urea is added to a mixed solution containing a saltof metal A and a salt of metal B while, if necessary, heating to causecoprecipitation.

The salt of metal A and that of metal B are not particularly limited.Useful salts of metal A or B include a sulfate, an oxysulfate, achloride, an oxychloride, a nitrate, an oxynitrate, and a carboxylate,with a sulfate, an oxysulfate, a chloride, and an oxychloride beingsuited.

A coprecipitation method in which a metal A alkoxide and a metal Balkoxide in a mixed solution is hydrolyzed simultaneously is alsosuitable.

The alkoxide of metal A and that of metal B are not particularly limitedand include, for example, a methoxide, an ethoxide, a propoxide and abutoxide of each metal.

The conditions causing coprecipitation are important, for they influencethe rate of coprecipitation, the shape of the coprecipitate formed, andthe like. Since the conditions vary depending on the starting material,the composition and concentration of the mixed solution, the kind of thecoprecipitate, the method for causing coprecipitation, and the like,suitable conditions are to be selected according to these factors.

The coprecipitate thus obtained is filtered, washed, and dried. Thedrying temperature is preferably relatively low, for example, a range offrom 100 to 200° C. being preferred. If the drying temperature exceeds600° C., the stability of the superabsorbent resin with time will bediminished.

It is preferable for the water-containing metal oxide to have a largespecific surface area. A preferred BET specific surface area is 100 m²/gor greater, particularly 200 m²/g or greater, for securing the stabilityof the superabsorbent resin with time.

The titanium alkoxide as compound (B-5) is not particularly limited aslong as it is an organotitanium compound having a reactive alkoxy group.Suitable titanium alkoxides include titanium tetramethoxide, titaniumtetraethoxide, titanium tetraisopropoxide, titanium tetrabutoxide,titanium tetrastearyloxide, titanium tetrakis(2-ethylhexyloxide), atitanium tetraisopropoxide polymer, a titanium tetrabutoxide polymer,diisopropoxybis(acetylacetonato)titanium,dibutoxybis(triethanolaminato)titanium, titanium tributoxide stearate,and titanium diisopropoxide distearate.

The sulfate of at least one metal A selected from the group consistingof titanium and zirconium as compound (B-6) includes titanium sulfate,titanyl sulfate, and zirconium sulfate.

The metal compound as component (B) preferably has a metal A content of0.001 to 1 part by weight, particularly 0.005 to 0.5 part by weight,especially 0.01 to 0.1 part by weight, per 100 parts by weight of asuperabsorbent resin as component (A).

If the metal A content is less than 0.001 part by weight, the resultingresin composition has insufficient gel stability. Even if the metal Acontent exceeds 1 part by weight, a further improvement is littleexpected.

The “superabsorbent resin” as referred to as a basis of contents isintended to mean a superabsorbent resin in its water-free dry state.

The chelating agent as component (C) is a chelating agent having a metalchelating ability, i.e., a compound having a bidentate or polydentateligand capable of bonding to a metal ion to form a metal chelate. Itshould be noted, however, that when compound (B-1) or (B-2) is used ascomponent (B), a hydroxy acid or a salt thereof is excluded from thechelating agent (component (C)).

Specific but non-limiting examples of the chelating agent as component(C) include water-soluble inorganic phosphoric acid compounds, such aspolyphosphoric acids, e.g., tripolyphosphoric acid, tetrapolyphosphoricacid, pentapolyphosphoric acid, pyrophosphoric acid, metaphosphoricacid, and polyphosphoric acid and salts thereof (e.g., Na salt or Ksalt); aminocarboxylic acid compounds, such asethylenediaminetetraacetic acid, 1,3-propanediaminetetraacetic acid,diethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid,L-glutamic acid diacetic acid, N,N-Bis(carboxymethyl)-L-glutamic acid,and hydroxyethylethylenediaminetriacetic acid, and salts thereof (e.g.,Na, K or ammonium salt); polyhydroxy compounds, such as glycol andglycerol; amine compounds, such as ethylenediamine, 1,10-phenanthroline,2,2′-bipyridine, and terpyridine; dicarboxylic acid compounds, such asoxalic acid or its salts (e.g., Na, K or ammonium salt); organicphosphorus compounds, such as aminotri(methylenephosphonic acid),1-hydroxyethylidene-1,1-diphosphonic acid,ethylenediaminetetra(methylenephosphonic acid),diethylenetriaminepenta(methylenephosphonic acid), and2-phosphonobutane-1,2,4-tricarboxylic acid, and salts thereof(e.g., Na,K or ammonium salt); tropolone or derivatives thereof, such asβ-thujaplicin, γ-thujaplicin, and salts thereof (e.g., Na, K or ammoniumsalt); compounds serving as a surface active agent, such as compoundrepresented by formula (I):

wherein R¹ represents an alkyl or alkenyl group having 6 to 30 carbonatoms; and M¹'s, which may be the same or different, each represents analkali metal ion, an ammonium ion or a hydrogen atom, such as a citricmonoalkylamide and a citric monoalkenylamide; compounds represented byformula (II):

wherein R² represents an alkyl or alkenyl group having 6 to 30 carbonatoms; and M²'s, which may be the same or different, each represent analkali metal ion, an ammonium ion or a hydrogen atom, such as amonoalkyl citrate or a monoalkenyl citrate; compounds represented byformula (III):

wherein R³ represents an alkyl or alkenyl group having 6 to 30 carbonatoms; and M³'s, which may be the same or different, each represent analkali metal ion, an ammonium ion or a hydrogen atom, such as analkylmalonic acid and an alkenylmalonic acid; compounds represented byformula (IV):

wherein R⁴ represents an alkyl or alkenyl group having 6 to 30 carbonatoms; and M⁴'s, which may be the same or different, each represent analkali metal ion, an ammonium ion or a hydrogen atom, such as anN-alkyl-N′-carboxymethylaspartic acid and anN-alkenyl-N′-carboxymethylaspartic acid; compounds represented byformula (V):

wherein R⁵ represents an alkyl or alkenyl group having 6 to 30 carbonatoms; and M⁵'s, which may be the same or different, each represent analkali metal ion, an ammonium ion or a hydrogen atom, such as amonoalkyl phosphate and a monoalkenyl phosphate; compounds presented byformula (VI):

wherein R⁶—CO— represents an acyl group having 6 to 30 carbon atoms; andM⁶'s, which may be the same or different, each represent an alkali metalion, an ammonium ion, a triethanolammonium ion or a hydrogen atom, suchas an N-acylated glutamic acid; and compounds represented by formula(VII):

wherein R⁷—CO— represents an acyl group having 6 to 30 carbon atoms; andM⁷'s, which may be the same or different, each represent an alkali metalion, an ammonium ion, a triethanolammonium ion or a hydrogen atom, suchas an N-acylated aspartic acid; and β-diketone compounds, such asacetylacetone, 4-hydroxybenzoylacetone and4-hydroxybenzoylmethane-t-butyl ketone. Among them preferred arewater-soluble inorganic phosphoric acid compounds, aminocarboxylic acidcompounds, organic phosphorus compounds, and surface active agentcompounds. Particularly preferred are tripolyphosphoric acid,polyphosphoric acid, ethylenediaminetetraacetic acid,1-hyhdroxyethylidene- 1,1-diphosphonic acid, monoalkyl phosphates,2-phosphonobutane-1,2,4-tricarboxylic acid, L-glutamic acid diaceticacid, N,N-Bis(carboxymethyl)-L-glutamic acid,hydroxyethylethylenediaminetriacetic acid, or salts thereof (e.g., Na, Kor ammonium salt).

Where a metal compound as component (B) is selected from compound (B-3),compound (B-4), compound (B-5) and compound (B-6), the chelating agentas component (C) further includes oxycarboxylic acid compounds, such astartaric acid, gluconic acid, citric acid, salicylic acid or saltsthereof (e.g., Na, K or ammonium salt).

The chelating agent as component (C) is preferably used in an amount of0.01 to 5 parts by weight, particularly 0.05 to 2 parts by weight, per100 parts by weight of a superabsorbent resin as component (A).

If the content of the chelating agent is less than 0.01 part by weight,the resulting resin composition has insufficient gel stability. Even ifthe content of the chelating agent exceeds 5 parts by weight, a furtherimprovement is little expected.

From the viewpoint of stability with time in the presence of water inwhich radical generating species such as L-ascorbic acid or salt aredissolved, component (C)/metal A is preferably 0.8-10, more preferably1.5-5 in terms of molar ratio.

Component (C) may be coordinated with metal A in the present invention.The compound (metal chelate compound) in which component (C) iscoordinated with metal A is referred to component (D) here.

The content of component (D) based on component (A) is preferably thetotal of preferable contents of metal A and component (C).

The superabsorbent resin composition of the present invention cancontain water in addition to the superabsorbent resin as component (A),the metal compound as component (B), and the chelating agent ascomponent (C), or the superabsorbent resin as component (A) and themetal chelate compound as component (D). This case includes anembodiment in which the superabsorbent resin is a water-containingpolymer and an embodiment in which the composition is in awater-containing gel state. The superabsorbent resin composition cancontain water within its absorptive capacity.

If desired, the superabsorbent resin composition can contain variousadditives, such as a water-soluble organic solvent, a surface activeagent, a salt, inorganic fine particles, a stabilizer, an antioxidant, areducing agent and/or an antiseptic. Water and these additives can beadded in a total amount of not more than 50% by weight based on thetotal weight superabsorbent resin composition.

A process for producing the superabsorbent resin composition whichcomprises mixing (A) the superabsorbent resin, B) the metal compoundcontaining at least one metal A selected from the group consisting oftitanium and zirconium, and (C) the chelating agent, or mixing (A) thesuperabsorbent resin and (D) a coordination compound in which component(C) is coordinated with metal A. The following methods (1) to (3) aregiven as examples. (1) A method comprising previously adding the metalcompound and the chelating agent to the preparation system of thesuperabsorbent resin. For example, in using a water-soluble vinylmonomer for providing a superabsorbent resin, the metal compound and thechelating agent are previously mixed with the water-soluble vinylmonomer, and the monomer is polymerized. (2) A method comprisingspraying an aqueous solution containing the metal compound and thechelating agent onto the superabsorbent resin either in a dry state or awater-containing condition and, if desired, drying the resin.

(3) A method comprising dry mixing the superabsorbent resin with themetal compound and the chelating agent, both in a dry state.

In carrying out the above methods, the metal compound and the chelatingagent may be mixed together beforehand and added to the superabsorbentresin (or the preparation system thereof) or be separately added.Further, the metal compound and the chelating agent may be added by thesame method or different methods selected from the methods (1) to (3).

While the water absorption capacity of the superabsorbent resincomposition according to the present invention is not particularlylimited, it is preferable for the composition to have a water holdingpower of 35 g/g or more, particularly 38 g/g or more, as measured inaccordance with a holding power measuring method by centrifugaldehydration hereinafter described.

In general, as the water absorption capacity of the superabsorbent resincomposition increases, the amount of resin required per article, e.g.,diaper, decreases, which contributes to reductions in the thickness ofthe diaper and manufacturing cost. However, as the water absorptioncapacity increases, the performance properties, such as gel stabilityover time, gel strength, and liquid permeability, are generally reduced.Therefore, resins exhibiting super absorptivity are difficult to applyto disposable diapers. To the contrary, the superabsorbent resincomposition according to the present invention has a relatively highwater absorption capacity having a holding power of 35 g/g or more andyet hardly undergoes such reductions in performance.

As stated above, the superabsorbent resin composition of the presentinvention is particularly useful as a water-absorbing material insanitary articles, such as absorbent articles, e.g., disposable diapersand sanitary napkins. Such absorbent articles comprise a water-permeabletopsheet, a water-impermeable backsheet and an absorbent memberinterposed between said topsheet and said backsheet. The absorbentmember can be made up of fluff pulp, i.e., ground wood pulp. Thesuperabsorbent resin composition of the present invention is used incombination with the fluff pulp either as a mixture with the fluff pulpor in the form of an independent layer on specific areas of a fluff pulplayer. The absorbent member can be prepared by heat treating a mixtureof a thermoplastic resin, fluff pulp, and the superabsorbent resincomposition of the present invention.

As mentioned above, because body fluids such as urine contain L-ascorbicacid or a salt thereof superabsorbent resin in a conventionalsuperabsorbent resin composition deteriorates by such substances presentin body fluids absorbed by the absorbent articles. To the contrary,where the superabsorbent resin composition of the present invention isused as a water-absorbing material of absorbent articles, deteriorationof the superabsorbent resin can be suppressed.

Moreover, the superabsorbent resin composition of the present inventionhas high gel strength and high liquid permeability after swelling, andis therefore suitable for use in absorbent articles, such as disposablediapers and sanitary napkins.

Unless otherwise indicated, all the percents and parts in the followingExamples and Comparative Examples are given by weight.

The test methods used in the Examples and Comparative Examples aredescribed below.

1) Measurement of Holding Power by Centrifugal Dehydration Method

A superabsorbent resin composition weighing 1 g was swollen with 150 mlof physiological saline (0.9% NaCl solution, produced by OtsukaPharmaceutical Co., Ltd.) for 30 minutes and put in a bag made ofnonwoven fabric. The bag and the contents were dehydrated in acentrifugal separator at 143 G for 10 minutes and weighed (overallweight). The holding power after centrifugal dehydration was calculatedaccording to equation (1).

Holding power after centrifugal dehydration (g/g)=[(overallweight)−(weight of nonwoven fabric bag)−(weight of superabsorbent resincomposition)−(residue of liquid in nonwoven fabric bag)]/(weight ofsuperabsorbent resin composition)  (1)

2) Evaluation of Gel Stability with Time after Swelling:

A superabsorbent resin composition weighing 1 g was swollen with 45 g ofphysiological saline containing 0.05% of L-ascorbic acid. The swollenresin composition was sealed in a screw tube and allowed to stand at 40°C. for 3 hours. The state of the swollen gel after the standing wasobserved by the eye to evaluate the gel stability with time. Theevaluation on gel stability with time was made in terms of gelflowability, stringiness, and shape retention according to a 4-graderating system shown in Table 1 below. Gel flowability was observed bytilting the screw tube up, stringiness was observed by stirring the gelwith spatula, and shape retention was observed by the eye after takingout of the screw tube. If the gel has flowability and stringiness, itmeans that the superabsorbent resin is degraded. If the degree of shaperetention lowers, it means that the superabsorbent resin is degraded.Superabsorbent resin compositions graded A or B are to be suitable foruse as a water-absorbing material in sanitary napkins, disposablediapers, sheets for adults, tampons, absorbent cotton, etc.

TABLE 1 Shape Grade Flowability Stringiness Retention A non-flowablenon-stringy unchanged B Slightly Slightly slightly Flowable Stringychanged C Flowable Stringy partly liquefied D Flowable Stringy largelyliquefied

Synthesis Examples for the superabsorbent resins, metal compounds andmetal chelate compound used in Examples and Comparative Examples areshown below. All the metal compounds and metal chelate compound inSynthesis Examples were obtained in the form of a solution.

SYNTHESIS EXAMPLE 1 Synthesis of Superabsorbent Resins (1) and (2)

In a 1000 ml four-necked flask equipped with a stirrer, a refluxcondenser, a dropping funnel, and a tube for nitrogen gas introductionwere charged 400 ml of cyclohexane and 0.625 g (0.5% based on theproduced polymer) of ethyl cellulose (Ethyl Cellulose N-100, produced byHercules Powder Co.) as a dispersant. Nitrogen gas was blown into themixture to driven out dissolved oxygen, and the contents of the flaskwere kept at 75° C.

In a separate flask 102.0 g of acrylic acid was diluted with 25.5 g ofion-exchanged water, and the solution was neutralized with 140 g of a30% sodium hydroxide aqueous solution while cooling from outside. To theaqueous solution was added a solution of 0.204 g (0.2% based on theacrylic acid) of potassium persulfate in 7.5 g of water. Nitrogen gaswas blown into the solution to remove dissolved oxygen. The contents ofthe flask were put dropwise into the above four-necked flask over 1 hourto carry out polymerization. After completion of the polymerization, thereaction mixture was azeotropically dehydrated by the use of adehydrating tube so as to adjust the water content of the resultingsuperabsorbent resin to 30 parts per 100 parts of the resin. Then asolution of 0.04 g (0.04% based on the acrylic acid) of polyglycerolpolyglycidyl ether (Denacol EX-512, produced by Nagase Kasei Kogyo K.K.)in 4 g of water was added thereto as a crosslinking agent, followed byallowing the mixture to react at 75 to 80° C. for 1 hour. After cooling,cyclohexane was removed by decantation to collect the superabsorbentresin (water-containing), which is designated superabsorbent resin (1).

The superabsorbent resin (1) was dried at 80 to 100° C. under reducedpressure of 50 Torr. The resulting resin is designated superabsorbentresin (2).

SYNTHESIS EXAMPLE 2 Synthesis of Superabsorbent Resins (3) and (4)

A water-containing superabsorbent resin, designated superabsorbent resin(3), was prepared in the same manner as in Synthesis Example 1, exceptfor replacing the ethyl cellulose used as a dispersant with 1.5 g of a25% aqueous solution of sodium polyoxyethylene lauryl ether sulfate(average mole number of ethylene oxide added =2) and increasing theamount of the polyglycerol polyglycidyl ether used as a crosslinkingagent from 0.04 g to 0.06 g.

The superabsorbent resin (3) was dried at 80 to 100° C. under reducedpressure of 50 Torr. The resulting resin is designated superabsorbentresin (4).

SYNTHESIS EXAMPLE 3 Synthesis of Metal Compound (5)

To an ice-cooled solution of 43.6 g of sodium gluconate in 150 g ofion-exchanged water was added dropwise 20 g of titanium tetrachlorideand mixed. After confirming that the solution turned clear, about 48 gof a 30% sodium hydroxide aqueous solution was added thereto dropwise toadjust the solution to pH 7. The resulting solution was clear andfaintly yellow and had a titanium content of 1.9% (calculated).

SYNTHESIS EXAMPLE 4 Synthesis of Metal Compound (6)

To 50 g of a titanium oxysulfate solution having a titanium content of4.9% (Titanyl Sulfate Solution, produced by Kisan Kinzoku K. K.) wereadded 32.2 g of citric acid monohydrate and 25 g of ion-exchanged waterand mixed. After confirming that citric acid completely dissolved, about104 g of a 30% sodium hydroxide aqueous solution was added theretodropwise while cooling from outside to adjust the solution to pH 7. Theresulting solution was yellow and slightly turbid. The titanium contentof the solution was 1.2% (calculated).

SYNTHESIS EXAMPLE 5 Synthesis of Metal Compound (7)

A solution of 23.3 g of titanium tetrachloride (TiCl₄) in 51 ml of waterand a solution of 20 g of zinc chloride (ZnCl₂) in 100 ml of water weremixed, and the mixture was diluted with water to make 5 liters. To themixed solution was added 71.5 g of urea, followed by stirring. Thesolution had a pH of 2. On heating the solution at 95° C. for 20minutes, a white coprecipitate was gradually formed. The heating wascontinued until the pH of the solution rose to 7. The coprecipitate wascollected by filtration and dried at 120° C. for 3 hours to obtain finepowder comprising water-containing metal oxide aggregates, which isdesignated metal compound (7).

The metal compound (7) had a BET specific surface area of 250 m²/g and aTi/Zn molar ratio of 46:54.

SYNTHESIS EXAMPLE 6 Synthesis of Metal Compound (8)

To a solution of 34.0 g of zirconium oxychloride octahydrate in 150 g ofion-exchanged water was added 43.6 g of sodium gluconate and dissolved.About 22 g of a 30% sodium hydroxide aqueous solution was added theretodropwise to adjust the solution to pH 7. The resulting solution wasclear and faintly yellow and had a zirconium content of 3.86%(calculated).

SYNTHESIS EXAMPLE 7 Synthesis of Metal Chelate Compound (9)

To 29.3 g of a titanium oxysulfate solution having a titanium content of4.9% was added 23.0 g of a 50% citric acid aqueous solution (produced byFuso Kagaku Kogyo K. K.) and mixed . To the aqueous solution was added20.6 g of a 60% of 1-hydroxyethylidene-1,1-diphosphonic acid aqueoussolution (Dequest 2010CS, produced by Nippon Monsanto K. K.) and mixed.While cooling the mixture outside, 27 g of a 30% sodium hydroxideaqueous solution was added thereto dropwise to adjust the solution to pH7. The resulting solution was pale yellow and slightly turbid. Thetitanium content of the solution was 1.5% and the1-hydroxyethylidene-1,1-diphosphonic acid content was 13.4%. To 2.4 g ofthe resulting solution was added 1.6 g of heavy water, followed by themeasurement of ¹³C-NMR with the use of Varian UNITY INOVA 300MB. Theresults evidenced that the peak of 72 ppm of the carbon atom located at1-position of 1-hydroxyethylidene-1,1-diphosphonic acid was split into68 to 72 ppm, and the peak of 20 ppm of the methyl group was split to 18ppm, and the 1-hydroxyethylidene- 1,1-diphosphonic acid was coordinatedwith titan.

EXAMPLES 1 TO 10

The superabsorbent resin (A) shown in Table 2 below was put in atwin-cylinder kneader, and the metal compound (B) shown in Table 2 andthe chelating agent (C) shown in Table 2 were added thereto in amountsshown (per 100 parts of the superabsorbent resin) either in a powderform or by spraying an aqueous solution thereof. The mixture wasthoroughly stirred to mix to obtain a superabsorbent resin composition.The amount of the metal compound (B) is in terms of metal A content.

Where superabsorbent resin (1) or (3) was used, the mixture was dried at80 to 100° C. under reduced pressure of 50 Torr to obtain asuperabsorbent resin composition. The resulting resin composition wasevaluated in terms of water holding power after centrifugal dehydrationand stability of swollen gel according to the above-described testingmethods. The results obtained are shown in Table 2.

Before making evaluation, coarse particles of 850 μm or greater wereremoved from the composition by sieving. The same evaluation wasrepeated in the following Example 11 and Comparative Examples 1-5.

EXAMPLE 11

The superabsorbent resin (1) was put in a twin-cylinder kneader, and themetal chelate compound (9) (14.9% aqueous solution) obtained inSynthesis Example 7 as component (D) was added thereto by spraying, suchthat the Ti content was 0.03 parts based on 100 parts of thesuperabsorbent resin. Then, the same procedure as that of Example 1 wasrepeated to obtain a superabsorbent resin composition, and the sameevaluation was repeated. The evaluation results show that the holdingpower after centrifugal dehydration was 43 g/g, and the stability of theswollen gel was graded A.

Comparative Examples 1 and 2

The superabsorbent resin (A) alone shown in Table 3 below was tested inthe same manner as in Example 1, with neither a metal compound ascomponent (B) nor a chelating agent as component (C) being addedthereto. The results obtained are shown in Table 3.

Comparative Examples 3 TO 5

The superabsorbent resin (3) as component (A) was mixed with either oneof a metal compound as component (B) and a chelating agent as component(C) as shown in Table 3 to prepare a superabsorbent resin composition inthe same manner as in Examples. Similarly to Examples, thesuperabsorbent resin composition was dried at 80 to 100° C. underreduced pressure of 50 Torr and tested in the same manner as inExamples. The results obtained are shown in Table 3.

*1,*2 and *3 in the following Tables 2 and 3 are as follows:

*1: Content of metal A (Ti or Zr) per 100 parts the superabsorbent resin(A).

*2: Titanium dioxide produced by Ishihara Sangyo, Kaisha (anatase type;average particle size: 7 nm; BET specific surface area: 320 m²/g)

*3: Titanium dioxide produced by Ishihara Sangyo, Kaisha (rutile type;average particle size: 40 nm; BET specific surface area: 40 m²/g)

TABLE 2 Performance Metal Compound (B) Chelating Agent (C) PropertiesSuper- Metal A Method Method Holding Swollen Example absorbent Contentof Amount of Power Gel No. Resin (A) Kind (part*¹) Addition Kind (part)Addition (g/g) Stability 1 (1) (5) 0.03 Sprayed Na tripoly-phosphate 0.510% soln. 43 A sprayed 2 (1) (5) 0.03 Sprayed Monoalkyl phosphate 0.5 4%soln. 43 A (C₁₂) sprayed 3 (1) (6) 0.04 Sprayed Na 1-hydroxy ethylidene-0.5 30% soln. 45 A 1,1-diphosphonate sprayed 4 (2) ST-01*² 0.4 Added asMonoalkyl phosphate 0.5 added as 42 A powder (C₁₂) powder 5 (3)TTO-55(N)*³ 0.6 Added as Na tripolyphosphate 1.0 10% soln. 43 B powdersprayed 6 (3) (7) 0.1 Added as Na 1-hydroxy-ethylidene- 0.5 30% soln. 44A powder 1,1-diphosphonate sprayed 7 (3) Titanium tetraiso- 0.1 SprayedNa 1-hydroxy-ethylidene- 0.5 30% soln. 40 A propoxide 1,1-diphosphonatesprayed 8 (3) Titanyl sulfate 0.04 Sprayed Na tripolyphosphate 1.5 10%soln. 42 A Sprayed 9 (4) (7) 0.2 Added as 2Na ethylenedi- 0.5 Added as43 A powder aminetetraacetate powder 10 (1) (8) 0.04 Sprayed Na1-hydroxy-ethylidene- 0.5 30% soln. 43 A 1,1-diphosphonate sprayed

TABLE 3 Performance Metal Compound (B) Properties Compara. Super- MetalA Chelating Agent (C) Holding Example absorbent Content Method of AmountMethod of Power Swollen Gel No. Resin (A) Kind (part*¹) Addition Kind(part) Addition (g/g) Stability 1 (2) — — — — — — 43 C 2 (4) — — — — — —47 D 3 (3) titanium 0.1  Sprayed — — — 41 C tetraiso— propoxide 4 (3)titanyl 0.04 Sprayed — — — 44 C sulfate 5 (3) — — — Na tripolyphosphate1.0 10% soln. 44 D Sprayed

As is apparent from the results in Tables 2 in view of Table 3, thesuperabsorbent resin compositions according to the present invention(Examples 1 to 11) which comprise components (A), (B) and (C), orcomponents (A) and (D) are superior to the comparative compositions inboth holding power after centrifugal dehydration and stability ofswollen gel.

What is claimed is:
 1. A superabsorbent resin composition comprising thefollowing components (A), (B) and (C): (A) a superabsorbent resin, (B) ametal compound containing at least one metal A selected from the groupconsisting of titanium and zirconium, and (C) a chelating agent in anamount of 0.01 to 5 parts by weight of said component (A), wherein saidchelating agent is selected from the group consisting oftripolyphosphoric acid, polyphosphoric acid, ethylenediaminetetraaceticacid, 1-hydroxyethylidene-l,1-diphosphonic acid, a monoalkyl phosphateand salts thereof.
 2. The superabsorbent resin composition according toclaim 1, wherein said metal compound as component (B) is a compoundobtained by mixing a hydroxy acid or a salt thereof with a polyvalentmetal salt comprising at least one metal A selected from the groupconsisting of titanium and zirconium or an alkoxide of said metal A. 3.The superabsorbent resin composition according to claim 1, wherein saidmetal compound as component (B) is obtained by hydrolyzing a polyvalentmetal salt comprising at least one metal A or an alkoxide of said metalA, in the presence of a hydroxy acid or a salt thereof, wherein saidmetal A is selected from the group consisting of titanium and zirconium.4. The superabsorbent resin composition according to claim 1, whereinsaid metal compound as component (B) is a water-containing metal oxidecomprising at least one metal A selected from the group consisting oftitanium and zirconium and at least one meatl B selected from the groupconsisting of zinc, aluminum, calcium, magnesium, and silicon.
 5. Thesuperabsorbent resin composition according to claim 1, which is capableof holding 35 g/g or more of physiological saline after being swollenwith physiological saline for 30 minutes and dehydrated bycentrifugation.
 6. A process for producing the superabsorbent resincomposition as claimed in claim 1, comprising the step of mixingtogether: (A) the superabsorbent resin, (B) the metal compoundcontaining at least one metal A selected from the group consisting oftitanium and zirconium, and (C) the chelating agent.